1. Field of the Invention
The present invention relates to an ophthalmological measuring apparatus, and more particularly relates to an ophthalmological measuring apparatus that derives an actual distance on an ocular fundus based on a fundus image on a screen.
2. Description of the Prior Art
With fundus cameras, there is a demand to be able to view a photograph taken of a fundus and derive the size of a lesion part, the distance on the retina from the lesion part to the macula lutea, and the like. This is because Japanese Patent Publication No. 2000-60893, for example, discloses a photodynamic therapy (PDT) that treats age-related macular degeneration by irradiating a spot of the lesion part with a laser beam, and the actual distance on the retina is needed in this therapy in order to form an appropriate laser spot. When providing treatment by irradiating a lesion part with a laser spot in this manner, it is a problem if the spot size of the beam to be shone is either too large or too small, and there is consequently a demand to form a laser spot of a prescribed size with high precision. Generally, it is said that the optimal spot size is the lesion maximum diameter plus 1 mm. Consequently, the maximum diameter of the lesion part must be accurately measured beforehand, and the beam spot size must be set to a size of the lesion part maximum diameter plus 1 mm.
The distance between prescribed points on a retina has conventionally been derived by assuming that the diopter of the affected eye is 0 diopters, setting a conversion magnification for determining the actual length from the taken image based on the specifications of the optical system of the fundus camera, applying that magnification to a taken image, and then deriving the length on the retina. Actually, there are individual differences that naturally arise in optical systems of fundus cameras, so that such differences must be eliminated. For this purpose, a model eyeball whose eye diopter is 0 diopters is used, and the conversion magnification is determined by using a simulation to derive a length at which the retina can be fully imaged in the mask when imaging the model eyeball with the fundus camera.
Nevertheless, the diopters of actual subject eyes vary, and the eye axial lengths, the corneal curvatures, and the like, also vary. In addition, there is a problem in that the operation of focusing the fundus camera varies the focal length (magnification) of the fundus camera, as is noted by Japanese Patent Publication No. 2003-225208. Accordingly, it is conventionally problematic to accurately derive the actual distance on an ocular fundus because there is considerable error in the distance calculation, which is based on the assumption that the affected eye diopter is 0 diopters.
On the other hand, it is necessary to recognize the position of the lesion part on the retina and to accurately derive the distance of that portion in order to form a laser spot having the appropriate size as mentioned above. To detect the position of the lesion part, a technology as disclosed in Japanese Patent Publication No. 1980-49778 is used that extracts the contour of an image based on, for example, brightness because the brightness at the lesion part varies.
In addition, Japanese Patent Publication No. 1994-125876 discloses a method wherein the papilla part and a pale part are also extracted from the fundus image based on the brightness, and Japanese Patent Publication No. 2003-310555 discloses a method wherein the average brightness of the papilla part is derived and the lesion part is thereby specified from the average brightness thereof.
It is therefore an object of the invention to provide an ophthalmological measuring apparatus capable of accurately deriving the actual distance on an ocular fundus.
It is another object of the invention to provide an ophthalmological measuring apparatus capable of easily specifying a prescribed region within a fundus image and capable of accurately measuring that region.